Britain appears to be getting a reputation as a safe haven for alleged mass murderers and torturers from abroad.

The case of Konrad Kalejs, who fled the UK on Thursday, has highlighted the issue.

Despite calls for him to be held pending further investigations into claims he was behind the killing of 30,000 people during World War II, he flew back to Australia where he is a resident.

Meanwhile, a Rwandan colonel accused of slaughtering 100,000 people is living as a refugee in south London. Genocide suspect Tharcisse Muvunyi came to Britain last year and has been granted asylum until 2002.

And Colonel Ian Henderson, a former head of the secret police in Bahrain who stands accused of torture, spent New Year on holiday in Devon, untroubled by the authorities.

Tharcisse Muvunyi: Alleged killer at large in London

Campaigners say all these cases show why Britain is a safe haven for suspected war criminals and torturers.

Legislation exists in Britain to bring such people to court.

The 1991 War Crimes Act gave the courts the right to try World War II criminals, but so far only one case has been successfully tried under this Act.

Last April 78-year-old Anthony Sawoniuk, a retired British Rail worker, was jailed for life for murdering 18 Jews while serving as a policeman in Nazi-occupied Belarus.

Under the 1988 Criminal Justice Act, torturers can be tried in Britain wherever their crimes were committed. But so far no-one has been brought to court under this legislation.

Evidence inadmissible

Dr James Smith of the Beth Shalom Holocaust Memorial Centre, told BBC News Online: "The problem in a case like that of Kalejs is that much of the evidence is [highly contestable] because the alleged incidents happened so long ago.

"And that's the problem with a lot of these war criminals - even if they were brought to trial they probably would not be convicted.

"However, I am disappointed that things were not taken a little further with Kalejs.

"They investigated the issue in the US, but further evidence has been uncovered and it would have been good at least to look into that."

He said allowing Col Muvunyi to stay in the UK while other Rwandan refugees were struggling to get in was "disgraceful".

"During the debate on the War Crimes Bill, the high cost of prosecuting war criminals was mentioned, but what else are we to ... worry about the money and allow the UK to be a safe haven for people that have committed mass murder?"

Anthony Sawoniuk: Convicted at a cost of £6m

One of the main problems for the UK is keeping suspects out.

After Mr Kalejs surfaced in the UK, Home Secretary Jack Straw called for a "full investigation".

Immigration officials reportedly missed the stamps in his passport declaring that he had been deported from America and Canada.

However, they are hampered by the fact there is no international register of people who have been deported from other countries.

Since the Kalejs case arose, the Parliamentary War Crimes Group has called on the government to help ensure immigration authorities set up a "watch list" of suspected war criminals.

However, the decision by the Metropolitan Police not to refer the case of Mr Kalejs to the Crown Prosecution Service highlights the basic weakness of Britain's war crimes legislation.

Prosecutions difficult

No war crimes legislation was enacted in the UK after the post-war Nuremberg trials because politicians and civil servants said no war criminals had entered the country.

The government only passed the relevant act in 1991 after mounting evidence and pressure from campaigners.

But in the case of alleged incidents that took place during the 1939-45 war, the War Crimes Act makes it hard to bring a prosecution.

Credible evidence is often difficult to find. Crucially, witnesses have to be able to identify clearly and uncontroversially the defendant and there must be proof that defendants were in command and not simply following orders.

Last year it emerged that Anton Husak, who was on a list of suspected war criminals supplied to the government by the Soviet Union in 1988, would not face charges.

Hundreds of investigations

Mr Husak, 78, who lives in west Wales, had been investigated over alleged involvement in Nazi war crimes. But the CPS said there was insufficient evidence to prosecute.

Since the 1991 Act came into force a special police unit of the Metropolitan Police has investigated 376 cases.

But in nearly a third of the cases, the suspects were already dead. In many more, the accused were too old or too senile to be interviewed.

Another 25 were established as innocent. Some names had been referred to the unit by malicious neighbours.

The CPS is still examining other cases, but with the end of the war more than 50 years ago, it's a race against time before suspects die of old age.